1. Field of the Invention
The present invention broadly relates to novel composite ceramic structures and to novel methods of making the same. In particular, the invention relates to composite ceramic structures having a polycrystalline matrix surrounding or embedding substantially inert filler materials and to methods of making the structures by "growing" an oxidation reaction product from a parent metal into a permeable mass of filler material.
2. Background
Traditional methods of preparing ceramic articles do not readily lend themselves to the preparation of ceramic matrix composite materials, especially fiber- and/or wire-reinforced ceramic composite structures. A composite structure is one which comprises a heterogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic composite structure typically comprises a ceramic matrix which encloses one or more diverse kinds of filler materials such as particulates, fibers, rods or the like.
Traditional methods of preparing ceramic articles involve the following general steps: (1) Preparation of material in powder form. (2) Grinding or milling of powders to obtain very fine particles. (3) Formation of the powders into a body having the desired geometry (with allowance for shrinkage during subsequent processing). For example, this step might be accomplished by uniaxial pressing, isostatic pressing, injection molding, tape casting, slip casting or any of several other techniques. (4) Densification of the body by heating it to an elevated temperature such that the individual powder particles merge together to form a coherent structure. Preferably, this step is accomplished without the application of pressure (i.e., by sintering), although in some cases an additional driving force is required and can be provided through the application of external pressure either uniaxially (i.e., hot pressing) or isostatically, i.e., hot isostatic pressing. (5) Finishing, frequently by diamond grinding, as required.
In the preparation of ceramic matrix composite materials, the most serious difficulties with traditional methods arise in the densification step, number (4) above. The normally preferred method, pressureless sintering, can be difficult or impossible with particulate composites if the materials are not highly compatible. More importantly, normal sintering is impossible in most cases involving fiber composites even when the materials are compatible, because the merging together of the particles is inhibited by the fibers which tend to prevent the necessary displacements of the densifying powder particles. These difficulties have been, in some cases, partially overcome by forcing the densification process through the application of external pressure at high temperature. However, such procedures can generate many problems, including breaking or damaging of the reinforcing fibers by the external forces applied, limited capability to produce complex shapes (especially in the case of uniaxial hot pressing), and generally high costs resulting from low process productivity and the extensive finishing operations sometimes required.
Additional difficulties can also arise in the body formation step, number (3) above, if it is desired to maintain a particular distribution of the composite second phase within the matrix. For example, in the preparation of a fibrous ceramic composite, the powder and fiber flow processes involved in the formation of the body can result in nonuniformities and undesired orientations of the reinforcing fibers, with a consequent loss in performance characteristics.
Other methods are also used as means for forming ceramic matrix composites. For example, the formation of a matrix structure by the reaction of gaseous species to form the desired ceramic (a process known as chemical vapor deposition) is employed currently for silicon carbide fiber-reinforced silicon carbide matrix composites. This method has met with only limited success, partly because the matrix deposition process tends to occur on all of the composite second phase surfaces at once, such that matrix development only occurs until the growing surfaces intersect, with the trapping of porosity within the body being an almost inevitable consequence. In addition, the rate of matrix deposition has been so low as to make such composites prohibitively expensive for all but the most esoteric applications.
A second non-traditional approach involves the infiltration of the composite particles or fibers with a flowable organic material containing the necessary elements to form the desired ceramic matrix. Ceramic formation occurs by chemical reaction on heating this material to an elevated temperature. Once again, limited success has been achieved, in this case because elimination of the large amounts of volatile materials (necessary constituents of the initial flowable infiltrant composition) during the heating process tends to leave behind a porous and/or cracked ceramic body.
Seufert (U.S. Pat. No. 3,437,468) discloses certain composite materials made by a reaction process with molten aluminum. However, the matrix constituent of these materials inherently contains a large amount of magnesium aluminate, a material of less desirable properties (for example, lower hardness) than certain other ceramics such as aluminum oxide. In addition, the process of the Seufert Patent requires that the ceramics be formed, in major part, by reaction of aluminum with magnesium oxide and silicon dioxide (in free or combined form) which reduces the flexibility of the process and dictates that substantial amounts of silicon (in addition to magnesium aluminate) will be present in the matrix of the final ceramic product.